1. Technical Field
The present invention relates to sonar systems. In particular, it relates to the sonar systems that determine sonar transmission frequency according to the ambient noise spectrum.
2. Background
Sonar technology originated as a tool for use in marine operations. Its original applications were particularly valuable to the military to improve the ability of naval forces to detect other ships, tend to determine their position and direction. Overtime, this technology has found numerous useful applications. For example, it can be used to measure the contents of closed liquid containers, such as fuel storage tanks, etc. Likewise, this technology found substantial use in the medical arts. Physicians seeking to monitor the condition of pregnant women routinely use ultrasound technology, which is based on sonar technology, to generate fetal images without endangering the fetus. In each of these applications, the quality of the reflected signal, and the subsequent value of the data produced therefrom, are often adversely affected by ambient background noise.
When used in a marine environment, the sonar system is bombarded by ambient noise from a variety of sources. For example, the ship transmitting the sonar signal produces noise across a wide frequency range. There may be other ships or structures in the nearby water that are also generate noise across a wide frequency range. Due to the interaction of natural underwater structures and the natural movement of water, a substantial amount of ambient noise may also be produced. As a result, underwater sonar systems operate in an environment that has high levels of ambient noise that is generated from a variety of sources, and is spread across a wide frequency range. It would be desirable to have a method of minimizing the effect of ambient noise on sonar systems used for underwater applications.
The prior art has made attempts to address this issue. One method has been to use a cable to tow one or more hydrophones behind a ship. The separation of the hydrophones from the ship reduces the vibrations that would otherwise be directly transmitted to the hydrophones by mechanical attachment to the ship. Likewise, the hydrophones can be shaped to minimize turbulence and thereby reduce noise from its own movement through the water. However, this type of device is fixed in terms of its transmission/reception frequency and is therefore susceptible to variations in ambient noise levels at those frequencies.
Another approach has been to use a single transmitter in combination with multiple hydrophone sensors. By tuning individual hydrophone sensors to either the transmission frequency or harmonics thereof, the system can selectively use the different frequency data to obtain improved directional information. This system is also fixed in terms of its transmission frequency. The various hydrophone sensors are also fixed to a particular frequency and are therefore susceptible to variations in ambient noise levels at those frequencies.
Yet another approach has been to use a dual transmitter system in which a low frequency transmitter and corresponding hydrophone sensor is used for extended range and a high frequency transmitter and corresponding hydrophone sensor is used for short-range precision. As was the case in the prior art systems discussed above, this system also uses a fixed frequency system that is susceptible to variations in ambient noise levels at those frequencies.
None of the prior art systems address the problem created by the dynamic nature of the marine environment. In particular, the sensitivity problem created by the constant changes in ambient noise levels at any given frequency is not addressed by any of the foregoing systems. It would be desirable to have a method of reducing sensitivity problems caused by ambient noise level changes at particular frequencies.
Another technological area in which sonar technology has been found useful has been the development of sonar-based measurement devices. In particular, sonar devices are often used as measuring devices to examine the level of the contents of fluid containers, and/or the rate of fluid flow within conduits. As was the case above in regard to marine systems, ambient noise will vary greatly from one location to another, and further, will vary greatly in a single location over the course of time. As a result, fixed frequency devices will have varying performance results depending on the ambient noise existing at the time of measurement.
In the medical arts, the use of sonar technology in conjunction with ultrasound equipment has provided medical professionals with the ability to perform noninvasive examination procedures for variety of medical conditions. As was the case above, conventional ultrasound equipment does not provide the ability to avoid noise problems through the use of dynamic frequency selection methods.
Sonar system designs have difficulty detecting small sonar echoes because they do not contain enough attributes to distinguish them from other sounds, such as ambient noise. All the sonar echo has is amplitude and frequency. Likewise, ambient noise can be easily mistaken for a sonar echo when it is composed of a similar frequency and has sufficient amplitude. Hence, it is important that the sonar echo have the largest possible amplitude in relation to noise with similar frequency in order to facilitate its detection. It would be desirable to have a sonar-based system capable of selecting the lowest possible ambient noise level for a selected frequency in order to maximize the signal to noise ratio of the received sonar signal.
While the sonar technology has been used for a variety of applications, the prior art has failed to provide a method of monitoring ambient noise for the purpose of dynamically varying the sonar signal frequency to match ambient noises with low noise amplitude levels in order to maintain a high signal to noise ratio.
The present invention solves the foregoing problems by providing a sonar system which improves performance in noisy environments by dynamically selecting a sonar transmission frequency which matches a low ambient noise frequency such that a high signal to noise level is obtained. The system automatically scans the ambient noise across the frequency band used for sonar transmission, selects a frequency with a low ambient noise level and dynamically changes the transmitted sonar frequency to the frequency, thereby creating an improved signal to noise ratio. A preferred method of implementing the invention is to use a sensor and an FFT spectrum analyzer to monitor the environment. The FFT spectrum analyzer may be programmable or hardwired. The FFT spectrum analyzer selects a sonar transmission frequency by determining the ambient noise frequency with a low amplitude based on specified criteria. The sonar transmission system, and its components such as receiver filters, converters, discriminators, etc., has a wide operable sonar frequency range so that the transmission frequency can be dynamically altered based on the selected sonar transmission frequency. The selection of a transmission frequency can be made every transmission cycle, or only predetermined timed basis. An alternative embodiment provides the option of dynamically varying transmission amplitude as well based on the detected ambient noise levels, for the purpose of prolonging the life of the sonar sensor or minimizing sonar-radiated emissions. Another alternative embodiment provides for avoidance of self-interference by switching to an alternative frequency each transmission cycle instead of waiting for the echoes to diminish.